1. Field of the Invention
This invention relates to rechargeable electrical batteries, and more particularly to a method and apparatus for controlling the charging profile of a battery.
2. Description of the Prior Art
General aviation aircraft have a battery which provides electrical energy to the aircraft electrical system. The battery is usually a rechargeable battery, such as a nickel-cadmium battery, a lead-acid battery or other type of rechargeable battery. This battery is also used to operate a starter motor to start the aircraft engine. After a battery powered engine start cycle, the aircraft battery is substantially discharged, and it is frequently desired to recharge the battery as quickly as possible.
In many aircraft applications, the battery is charged using a constant voltage supplied directly from the aircraft DC bus. Constant voltage charging has several advantages. Since the battery is connected directly to the bus, it does not require additional hardware such as battery chargers. It does, however, inherently provide a high initial inrush of current into a discharged battery after an engine start. The charge current is initially quite large. After the initial high inrush current, a smaller charging current, determined by the bus voltage, continues to recharge the battery. In order to accommodate the battery's allowable charge current, the bus voltage must be set to a voltage value so that the initial inrush current is not too high. However, this lowers the level of the subsequent smaller charging current, and this relatively small charge current results in a fairly long battery recharge time.
The constant potential method also has other disadvantages. As the desired battery charge voltage varies, it is difficult to have one optimal voltage setting for all conditions. If the bus voltage is set too high, battery overcharging will occur, or if the bus voltage is set too low, the battery will not be charged to capacity. Large charging currents can cause generator overloads. Overcharging can lead to excessive heating of the battery, and heat, whether generated internally or external, is a prime source of battery degradation.
Overcharging can also lead to a phenomenon know as thermal runaway. Thermal runaway is a condition where a battery on a constant potential charge, typically at elevated temperature, will destroy itself through internal heat generation. The heat is generated as a result of overcharge currents. An increase in temperature causes a battery voltage drop which in turn leads to an increase in overcharging currents. This is a regenerative situation where the current and temperature then rapidly rise until the battery is destroyed.
Some control over battery charging and battery over-temperature protection has been accomplished in the past using dedicated battery chargers and temperature monitors. This equipment is used to eliminate generator overloads caused by high battery charge currents. However, the addition of these separate pieces of equipment increases the overall weight of the aircraft and the complexity of the electrical system.
More recently, the inventors herein have developed a microprocessor-based generator control unit. This unit is fully disclosed in co-pending U.S. patent application Ser. No. 08/131,196, assigned to the assignee of the present invention.
It would be desirable to implement the functions of battery charging control in the generator control unit which already controls and monitors many of the aircraft electrical system parameters, including parameters which could be used for controlling battery charging.